Ultrasonic Temperature Control and Measurement in Micro-Fluidic Channels H. Jagannathan , G.G. Yaralioglu, A.S. Ergun, and B.T. Khuri-Yakub Edward L. Ginzton Laboratory, Stanford University, Stanford, CA 94305-4088 ABSTRACT This paper describes the design and operation of a system used for the accurate control and measurement of temperature in micro-fluidic channels. Ultrasonic transducers are used for both heating and measuring the temperature of fluids inside the channel. Heating is performed by exciting the transducer with a tone burst at a single frequency. The temperature measurement is done through non-invasive means by monitoring the velocity of a sound propagating through the channel. The whole system is automated for data collection using Labview. The system developed has the advantages of easy integration, simple operation and design along with a wide application domain. The same ultrasonic transducer can be used for both heating and temperature measurement leading to possibilities of closed loop temperature control in micro-channels. The system requires milli-watts of power for heating and has a nano-second response time for temperature measurement with an accuracy of 0.1 degrees. Keywords: Ultrasound, piezoelectric, CMUT, micro-fluidics, temperature measurement, heating 1. INTRODUCTION The growth in the field of micro-fluidics has underscored the need for improved measurement and actuation techniques of fluids in micro-channels. Current techniques of sensing fluids in micro-channels include the use of lasers, fluorescence microscopy, and image processing by the use of cameras. These techniques involve bulky instruments which result in problems during system integration. The actuation of fluids is done through mechanical pumping or by the moving of micro-machined components such as switches or valves. Heating is commonly done by using polysilicon resistors fabricated inside the channel. While ultrasound theory is commonly used to study fluid flow in large pipes and tubes, it is rarely applied in the field of micro-fluidics. Our work emphasizes the use of ultrasound to achieve useful operations in micro- channels [1]. Both piezoelectric transducers and Capacitive Micromachined Ultrasonic Transducers (CMUTs) are used for this purpose. The piezoelectric devices operate at a center frequency of 400 MHz in air and have been used in our experiments. Capacitive Micromachined Ultrasonic Transducers have also been fabricated operating at a center frequency of 50 MHz. The channels are made of either PDMS (polymethylsiloxane) or glass. The control and monitoring of temperature in micro-channels is a very important task. The ability to measure and control temperature precisely will result in the monitoring of reaction rates between two or more chemicals, the measurement of flow rate or even the state of the fluid or sample. Many bio-medical procedures, Polymerase Chain Reaction (PCR) among others involve cycling of temperatures. The ability to control and monitor the temperature in micro-channels would enable the same procedures to be carried out in small and portable devices. 2. THE ULTRASONIC TRANSDUCERS Traditional ultrasonic transducers are made using piezoelectric materials that possess a property of producing charge proportional to the applied stress or vice versa. Today, a large variety of ultrasonic transducers are available. Some of these are made from new technologies that were not available in the past. Each type of transducer has particular advantages depending on its field of application. For example, the frequency of operation of transducers is generally a strong function of the thickness of the material deposited. This is generally the main reason for piezoelectric ceramics like PZT to be used for relatively low frequency applications. Whereas the Microfluidics, BioMEMS, and Medical Microsystems, Holger Becker, Peter Woias, Editors, Proceedings of SPIE Vol. 4982 (2003) © 2003 SPIE · 0277/286X/03/$15.00 243